1. Technical Field
This invention relates to a control circuit for a DC motor, and more particularly relates to a soft switching control circuit for a DC motor.
2. Description of Related Art
FIG. 1 is a circuit diagram of a typical DC motor driving circuit. The DC motor driving circuit 180 has four switches M1, M2, M3, and M4 composing an H-bridge circuit to drive the motor. The operation of H-bridge circuit can be divided into two distinct conduction states. In the first conduction state (state I), the switches M1 and M4 are turned on. In the second conduction state (state II), the switches M2 and M3 are turned on.
This H-bridge circuit operates in the first conduction state and the second conduction state alternatively to maintain the driving force for the motor. FIG. 2 shows the waveforms of the switching signals A, B, C, and D of the switches M1, M2, M3, and M4 and the coil current I1 on the motor coil. The coil current I1 flowing to the right is defined as a positive current.
The distance between the magnet and the coil is varied attending with the rotation of the motor. Right before reaching the state change time, the movement of the magnetic pole enhances the coil current I1 because of the significant variation of induced electromotive force (emf) generated between the magnet and the coil. However, right after the state change time, the coil current drops suddenly because the state of switches M1, M2, M3, and M4 are changed. The sudden change of coil current may result in the creation of acoustic noise. In addition, since the magnetic field generate by the motor coil lacks the ability to enhance the driving force when the magnetic pole is adjacent to the motor coil, the additional coil current (as indicated by the shaded region, which can be regarded as idle current) generated adjacent to the state change time cannot effectively contribute for driving the motor. The magnetic field generated by the idle current is a waste of power.
The lower part of the diagram in FIG. 2 also shows a typical method dealing with the above mentioned problem. As shown, an absolute value signal Vabs is generated according to the Hall signals H+ and H−. The absolute value signal Vabs and a fixed threshold voltage Vth are compared to form a cyclic signal to define a fixed adjusting time period. The adjusting time period is utilized for adjusting the timing of the rising edge and the falling edge of the original switching signals A and B so as to generate the new switching signals A1 and B1 for discharging the motor coil at a time earlier than that defined by the original switching signal B and charging the motor coil at a time later than the original switching signal A so as to prevent the generation of idle current.
However, this method lacks the flexibility to deal with the variations of motor parameters such as coil current, rotation speed, and etc., and the fixed adjusting time may be too large or too small. If the adjusting time period is too large, a significant time gap with no coil current is generated near the state change time. During the time gap with no coil current, the driving circuit has no driving force and may influence the stability of steady rotation of the DC motor. In contrast, if the adjusting time is too small, the purpose of soft switching control cannot be fulfilled.